1. Field of the Invention
This invention relates to speed reducers, and particularly those with very low ratios, and a unique transmission which is able to transmit higher torque levels and provide more efficient motion than prior art transmissions. This transmission could be applicable for use in a helicopter gear train.
Transmissions are utilized to transmit rotation for a variety of purposes. The term "transmission" as utilized in this application, does not specifically refer a vehicle transmission, although it would extend to such transmissions. Rather, this invention extends to any system wherein a source of movement is transmitted through a driving member to move a driven member. One potential applications for this transmission is a helicopter rotor drive. As is known, one of the biggest problem, associated with helicopter rotor drives is noise. When compared to conventional non-parallel shafts gear transmissions, worm/gear type transmissions generate minimum noise. However, low efficiency and torque capacity associated with prior art worm/worm gear transmissions prevented their use in helicopter power transmission systems.
2. Description of Prior Art
Worm/wormgear transmissions, in particular double enveloping speed reducers or Cone drive worm/worm gears, are well known in the mechanical power transmission field. The worm gear is driven by the rotation of the worm with which it meshes. The rotational speed of the associated shaft of the worm gear is a function of the number of teeth on the worm gear and the number of threads on the worm. The worm may be single or multiple threaded. The prior art worm/wormgear transmission had a worm gear with 24 or more teeth. In particular, the American National Standard "Design of Industrial Double-Enveloping Wormgears" (ANSI/AGMA -6030-C87) recommend 24 as the minimum number of gear teeth. Furthermore, the enveloping angle of any well-known worm for one revolution of it thread is not more than 15.sup.0.
In all standard double enveloping worm/worm gear transmissions, the enveloping worm gear has a surface that is generated by the profile of an enveloping thread of the worm. The term "Generated" describes how the profile of worm gear tooth can be defined. It could utilize mathematical calculations defining the profile from equations of the surface of the enveloping worm thread; hobbing of a gear blank by a tool, having the profile of the worm thread; or via computer modeling, where the profile of a 3D solid worm gear is cut by the profile of a 3D solid worm thread. Conventional enveloping worm/worm gear transmissions did not use worm gears with less than 24 enveloping type gear teeth due to the undercut on the root of the tooth. The enveloping angle of the worm is the angle of area contact between the threads on the worm and worm gear teeth. For a worm with one revolution of thread, the maximum number of engaging gear teeth is 2. As is known the enveloping thread angle can be calculated by equation EQU .phi.=360.sup.0* /N
Where N is the number of worm gear teeth.
Thus, for 24 teeth, .phi..sub.24 =15.sup.0. Likewise, for 12 teeth .phi..sub.12 =30.sup.0. This angle is also angular worm gear pitch.
If we use a two thread worm, then the enveloping angle should be twice as big. For example, an enveloping angle of a single thread worm for a worm gear with 6 teeth is 60 degrees while the enveloping angle of double thread worm for a worm gear with 6 teeth is 120 degree.
According to the Popov (U.S. Pat. No. 4,047,449) Patent, in order to increase the amount of tooth contact by increasing the number of teeth in actual contact, the enveloping angle should be bigger. However the Popov patent teaches of an enveloping angle which is still less than 15 degrees.
In all standards enveloping worm/worm gear transmissions, only a line contact exists between the worm thread and the worm gear teeth.
Moreover, standard double enveloping worm/worm gear transmissions have been used only for ratios of 5 and more. Due to such high ratios, is has been considered impractical to use the worm gear as the driven member and the worm as the driving member to transfer power from worm gear to worm.
The McCartin patent (U.S. Pat. No. 3,597,990) discloses a transmission with enveloping worm meshed with threaded followers. Thus, the McCartin gear with threaded followers is not able to have an envelope profile. Profiles of standard enveloping worm gear teeth usually have a profile generated by hobbing. However, the McCartin thread followers could not be made by hobbing or by generation of the worm thread profile. However, the McCartin drive is used for indexing motion and does not have a self-lock feature. McCartin patent can use one thread with more than two revolutions for accurate indexing. McCartin has line of contact in the middle of output rotary member. For one revolution of thread the McCartin device has a contact ratio of less than one which, in most applications is not practical. In McCartin patent, the enveloping of output rotary member (thread followers) is not possible and therefore the design has limited capacity. Finally the McCartin patent has ratatable members, which rotate very fast because their diameter is smaller than the pitch diameter of the worm. As such, these ratatable members have low reliability, are more expensive and produce extra noise.
In the Winzeler patent (U.S. Pat. No. 3,535,948), it is a split type worm gear that eliminates backlash by two halves involved in transferring torque.